Aggregates and Disrupted Dynein-Dependent Trafficking in ALS

Amyotrophic Lateral Sclerosis (ALS) is an adult-onset neuro-degenerative disease. The death of motor neurons leads to progressive paralysis of voluntary muscles, making patients unable to control their movements, and ultimately leads to paralysis of respiratory muscles and death. So far there is no effective treatment available. It is not clear how motor neurons die in ALS-patients. 10% of the ALS-cases are familial, and so far mutations in 6 genes have been identified in ALS-families. Mutations in the gene for SOD1 were the first to be discovered and account for up to 20% of familial ALS-cases. Most likely, mutant SOD1-protein is misfolded and accumulates in insoluble aggregates, that are toxic to motor neurons. In the studies described in this thesis, we use transgenic mice that express human mutant SOD1, and that develop paralysis and other pathological changes resembling ALS. We have studied changes in motor neurons in the spinal cord of these SOD1-ALS mice in great detail, and show that the appearance of aggregates is a very early phenomenon in these cells. In addition, motor neurons develop accumulations resembling a traffic jam, and a fragmented Golgi apparatus, an indication for disrupted cellular transport. Supporting cell-types, as astrocytes and microglia, are also affected in the spinal cord of SOD1-ALS mice. To study which cell-types are primarily responsible for the death of motor neurons, we developed transgenic mice with a restricted expression of mutant SOD1 in neurons. These mice develop similar pathological changes as SOD1-ALS mice, indicating that mutant SOD1 acting solely in neurons is sufficient to induce an ALS-like disease. Several studies have indicated that disruptions of axonal transport could cause motor neuron death. We have generated transgenic mice with a disruption of the retrograde microtubule motor dynein/dynactin, by expressing the dominant-negative linker protein BICD2-N in neurons. These mice have impaired retrograde axonal transport and develop cellular changes that resemble those found in SOD1-ALS-mice, like a disrupted Golgi apparatus and neurofilament accumulations. However, up to two years the mice are healthy and do not show any forms of paralysis. Moreover, if we cross these BICD2-N mice with SOD1-ALS mice, animals develop paralysis at a later age and survive longer, indicating that a disruption of retrograde transport could be beneficial for SOD1-linked ALS. Mutations in the gene for VAPB were most recently discovered in ALS-patients. We show that this relatively unknown protein is expressed at high levels in motor neurons in humans and mice and localizes to the Endoplasmic Reticulum. The ALS-linked mutation mislocalizes VAPB into insoluble cytoplasmic aggregates, that are toxic to neurons in culture. Moreover, mutant VAPB loses its ability to bind to Lipid Transfer Proteins, suggesting that this mutation could cause a disruption in lipid homeostasis in motor neurons. In sum, this thesis provides an overview of the current knowledge on the role of aggregates and trafficking in ALS, and a new view on the role of glia, retrograde transport and lipids in this disease.

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